In general, existing drying methods include a hot-air drying method, a cold-air drying method, and a freeze vacuum drying method, and a drying method is selected according to the type of articles to be dried, considering economy and experiences.
In spite that the cold-air drying method ensures the taste, odor, quality, and function of a food, its uses are restricted by high costs, and technical investment and researches for the cold-air drying method have not been sufficient due to the small size of the cold-air drying market, hampering development of the technology. The cold-air drying technology has developed with its specialty, but the desiccant drying technology is nearly an exploitation field and its importance has not been recognized. Thus, combination of the two technologies is a big change and is expected to be required in the market.
The drying characteristics of articles include constant rate drying and falling rate drying. In an existing arranged cold-air dryer, since the cooling heat of the cold-air dryer is used to remove the moisture load and the heating load used to vaporize moisture in dried articles is removed by the condensation heat of a condenser in the constant rate drying section, the difference in the absolute humidity of an inlet and an outlet of the evaporator, enhancing efficiency. However, in the falling rate drying section, since the cooling load is large as compared with the cooling heat of the cold-air dryer and the difference in absolute humidity is small as time lapses, the drying time becomes longer, making it difficult to efficiently use energy. Thus, it is impossible to operate the dryer below a predetermined humidity.
The constant rate drying state is nearly a state where moisture is vaporized from a water surface. That is, the drying speed is constant at the beginning stage of the drying operation.
If the constant rate drying operation is completed, the drying speed and (the amount of vaporized moisture*the percentage of water content) are linearly (proportional) reduced in the falling rate drying operation. It is called a first falling rate drying stage.
Next, the drying speed, the percentage of water content, and the amount of vaporized moisture are changed in a curved manner. It is called a second falling rate drying stage.
Meanwhile, Korean Patent No. 10-0614280 discloses “a low dew point cooling/dehumidifying system and a cooling/dehumidifying method”.
The patent relates to a low dew point cooling/dehumidifying apparatus which includes a compressor, a condenser, an expansion valve, an evaporator, and a blower to allow air to pass through the evaporator, and includes a refrigerant liquid valve installed on a freezing cycle, a pressure detector installed around the evaporator, a reheating coil connected to an exterior air condenser in parallel to pass the drying chamber air which has passed through the evaporator, a hot gas valve installed at an inlet side of the reheating coil, a condenser valve installed at an inlet side of the condenser, a low pressure switch installed between the compressor and the evaporator, and an evaporator temperature sensor connected to the evaporator, wherein the air which has passed through the evaporator passes by the reheating coil.
However, the patent has the following disadvantages.
That is, the cold-air drying operation at a dew point of below 5 degrees Celsius is almost impossible due to stacked frost, and since the dehumidifying load becomes larger than the cooling load to maintain a low humidity in the cold-air drying operation even when the stacked frost heat is used at a low temperature operation, the dehumidifying efficiency of a freezer becomes lower, considering the drying load, and the operation costs increase.
Although the cold-air drying method is advantageous as the condensation heat of the condenser can be utilized by 100% during the constant rate drying operation, the falling rate drying operation of dried articles becomes dull. In addition, since the actual amount of dried moisture significantly lowers as compared with the cooling efficiency, operation costs increase as the drying time of the dried articles becomes longer.